This invention relates to a method and apparatus for simply and rapidly determining an ammonia concentration of a gas by reacting nitrogen oxides with ammonia, and obtaining the ammonia concentration from a change in concentrations of the nitrogen oxides.
Heretofore, (1) neutralization-titration method, (2) indophenol method, (3) Nessler method, (4) solution electro-conductivity method, (5) infrared absorption method, and (6) detector tube method have been available as analytical methods for ammonia, as set forth in Japanese Industrial Standards. However, the neutralization-titration method (1) has such disadvantages that the method is influenced with a presence of basic and acidic gas components together with ammonia, and the method is not appropriate for a low ammonia concentration range, for example, less than 100 ppm.
The indophenol method (2) and the Nessler method (3) have a high analytical precision, but require at least one hour for conducting an absorption operation and color-developing operation, and therefore fail to meet a requirement for rapid and continuous analysis.
The solution electro-conductivity method (4) needs various complicated pretreatments, if a sample contains sulfuric acid, halogens, etc. which can change the electro-conductivity, and therefore lacks in rapidity.
The infrared absorption method (5) has a low detection sensitivity, and therefore is not suitable for the analysis in a low concentration range. The detector tube method (6) can only teach the concentration of a sample in a rough value.
On the other hand, analysis of ammonia in the atmosphere or other industrial flue gases requires an accurate and rapid determination in a relatively low concentration range and in the presence of various coexisting gas components, and also requires a continuous automatic analysis. Therefore, the analytical methods for ammonia, as mentioned above, cannot satisfy these requirements. Especially important is an analytical method for ammonia in apparatuses for removing nitrogen oxides in boiler flue gas or gases evolving from the nitric acid industry by reducing nitrogen oxides with ammonia. In such apparatuses, a flue gas containing nitrogen oxides (which will be hereinafter referred to as NO.sub.x) is admixed with an ammonia gas, and brought into contact with a catalyst at an elevated temperature to reduce NO.sub.x to nitrogen.
If an insufficient amount of the ammonia gas is added thereto, NO.sub.x cannot be completely reduced.
On the other hand, if an excessive amount of the ammonia gas is added thereto, the ammonia gas will leave the NO.sub.x removal plant as an effluent, possibly causing a secondary air pollution. The excessive amount of the ammonia gas also reacts with other components of the flue gas to form various ammonium salts, which have a possibility to clog the piping system or heat exchanger, or the like in the NO.sub.x removal facility. Therefore, accurate, rapid, and continuous quantitative analysis of ammonia and control of the amount of ammonia to be added are required.
Heretofore, the amount of ammonia to be added is determined by measuring the flow rate of a flue gas, and concentration of NO.sub.x or ammonia.
Suppose that:
C.sub.NO : Concentration of NO.sub.x in flue gas (ppm) PA0 F : Flow rate of flue gas (Nm.sup.3 /hr.) PA0 .alpha. : Moles of ammonia reacting with one mole of NO.sub.x
an amount D, a feed rate of ammonia necessary for reducing NO.sub.x in the flue gas, is given by the following formula: EQU D = (C.sub.NO .times. F .times. 17 .times. 10.sup..sup.-6 .times. .alpha. ) .div. 22.4 (kg/hr.)
Among the items to be measured, the concentration of NO.sub.x in the flue gas can be measured by analytical methods, such as chemi-luminescence method, constant potential electrolytical method, etc. The flow rate of the flue gas can be measured by calculation on the basis of boiler loads, or by measuring a gas flow rate by means of a pitot tube, orifice, etc., but the calculated or measured values are generally not so accurate. The value .alpha. is obtained from said value of NO.sub.x and an alalytical value of ammonia by light absorption method, etc., but it is difficult to carry out accurate and continuous analysis of ammonia. Even if the amount of the reducing agent to be added is exactly calculated, it is not so simple to control the amount of ammonia to be added with a high precision since the amount of NO.sub.x in a flue gas changes with time in the actual plant, or the reaction conditions of the No.sub.x removal plant change. That is, the accurate and continuous determination of the amount of ammonia will make the operation of the NO.sub.x removal facility very convenient.